To meet the bandwidth requirements of current and future high speed applications, state-of-the-art telecommunication components and systems must provide a host of sophisticated signal processing and routing functions, in both the optical and electronic domains. As the complexity level increases, integration of more functions and components within a single package is required to meet system-level requirements and reduce the associated size and cost of the end system. It has been recognized for some time that the integrated circuit devices, processes and techniques that revolutionized the electronics industry can be adapted to produce optoelectronic integrated circuits. In typical optoelectronic integrated circuits, light propagates through waveguides of high refractive index materials such as silicon, gallium arsenide, lithium niobate or indium phosphide. The use of high-index materials enables smaller size devices, since a higher degree of mode confinement and tighter bends may be accommodated. While all transmitter, signal processing and receiver functions may be incorporated in a single optoelectronic integrated circuit, the system may also be constructed from more than one package, referred to in the art and hereinafter as “hybrid optoelectronic integration”, or “multi-module optoelectronic integration”.
To enable many of the applications for telecommunications systems, it is necessary to consider the optical device performance when different wavelengths are launched into the device. For a number of applications, the wavelengths of interest fall in a continuous band delimited by a minimum wavelength λmin and a maximum wavelength λmax. As an example, many wavelength-division-multiplexed (WDM) systems operate over a wavelength band defined as the “C-band” that roughly corresponds to a wavelength band from 1525-1570 nm. This same technique can be expanded to cover L-band (wavelength band from 1570-1620 nm), S-band (wavelength band from 1480-1520 nm), as well as other exemplary wavelength bands.
In more specific terms, there are two different classes of sources that are desired to be able to couple into an optical device: variable-wavelength sources and multiple-wavelength sources. A variable-wavelength source is defined as a source that only emits a narrow band of wavelengths, centered around a wavelength λC, where λC can be varied via a tuning mechanism. One exemplary embodiment of a variable-wavelength source is a tunable laser module, operating with a center wavelength λC that can be tuned over the C-band wavelength range; similar modules would provide tuning over other exemplary bands. The typical linewidth of such an exemplary source is quite narrow, on the order of 0.05 pm, and the shift in λc with temperature is on the order of ±0.05 nm. A multiple-wavelength source is defined as a source that simultaneously emits several wavelengths centered on a wavelength λC. One exemplary embodiment of a multiple-wavelength source is an optical fiber input carrying a WDM signal, operating over the band of 1530-1565 nm, with a separation of 0.4 nm (50 GHz) or 0.8 nm (100 GHz) between adjacent wavelengths.
In the prior art, techniques referred to as “butt coupling” or “end-fire coupling” have commonly been used to couple light from external sources into optical waveguides. Specifically, end facets are cleaved on the waveguides, and optical fibers (which may be lensed for focusing purposes) are aligned to the input and output waveguide facets. While these coupling methods are relatively wavelength-insensitive, the insertion loss associated with such an arrangement increases substantially as the waveguide thickness drops below 2.0 μm. For sub-micron thick waveguides, the dimensional mismatch between the input/output beams and the thickness of the waveguide results in an insertion loss that is unacceptable for many applications.
To improve the insertion loss associated with wavelength-insensitive coupling into relatively thin waveguides, a variety of tapered structures that gradually reduce the beam size from its large external value to a dimension that is more closely matched to the waveguide have been proposed. Some examples include tapers that neck down in one or two dimensions from the external beam to the waveguide, and an “inverse taper” or “nanotapers” that has a narrow tip (often on the order of 100 nm wide) at the external beam, which then increases laterally in dimension until it matches the waveguide width. Of these examples, only the inverse taper has been successfully used to couple an appreciable amount of light into sub-micron waveguides. However, the inverse taper arrangement suffers from a number of drawbacks, such as: (1) a rapid increase in insertion loss with sub-micron misalignments; (2) the need for specialized techniques, such as e-beam lithography, to fabricate the nanotapers; and (3) the need for additional waveguiding structures prior to the tip of the nanotapers if the end of the tip is not coincident with the edge of the input facet.
Thus, a need remains in the art for providing a robust and manufacturable arrangement that is capable of coupling various types of multiple wavelength external sources into a relatively thin, planar silicon waveguide.